Method for purifying combustion exhaust gas

Abstract

The method for purifying combustion exhaust gas according to the present invention utilizes a NH 3 decomposing catalyst. The NH 3 decomposing catalyst in the present invention is capable of converting substantially all of the NH 3 in the combustion exhaust gas to N 2 when the air-fuel ratio of the exhaust gas is lean and the temperature of the catalyst is within a predetermined optimum temperature range. Further, when the exhaust gas contains NO x in addition to NH 3 , the NH 3 decomposing catalyst is capable of reducing the NO x in the optimum temperature range even though the air-fuel ratio of the exhaust gas is lean. In the present invention, the conditions of the exhaust gas containing NO x are adjusted before it is fed to the NH 3 decomposing catalyst in such a manner that the temperature of the exhaust gas is within the optimum temperature range and the air-fuel ratio of the exhaust gas is lean. Further, NH 3 is added to the exhaust gas before it is fed to the NH 3 decomposing catalyst. Therefore, a lean air-fuel ratio exhaust gas, at a temperature within the optimum temperature range, which contains both the NO x and NH 3 is fed to the NH 3 decomposing catalyst, and the NO x , as well as the NH 3 , in the exhaust gas is completely resolved by the NH 3 decomposing catalyst.

We claim: 
     
       1. A process for resolving NH 3  included in a combustion exhaust gas including the steps of: providing an NH 3  decomposing catalyst in an exhaust passage, wherein the catalyst resolves NH 3  when the temperature of the catalyst is within a predetermined temperature range and an atmosphere in which the catalyst is located is an oxidizing atmosphere, and the catalyst converts NH 3  to NO x  when the temperature of the catalyst is higher than said predetermined temperature range and the atmosphere is an oxidizing atmosphere, and wherein NH 3  passes through the catalyst when the temperature of the catalyst is lower than said predetermined temperature range and the atmosphere is an oxidizing atmosphere, so that exhaust gas flowing out of the NH 3  decomposing catalyst is substantially free from NH 3  and NO x  ;   directing a flow of exhaust gas including NH 3  so that the exhaust contacts the catalyst.   
     
     
       2. A process for resolving pollutants in exhaust gas of an internal combustion engine comprising: contacting, in a reducing atmosphere, the exhaust gas with an NH 3  synthesizing catalyst which, in a reducing atmosphere, converts NO x  components in the exhaust gas to NH 3  ;   adjusting an oxygen content and a temperature of the exhaust gas after it contacts the NH 3  synthesizing catalyst so that the exhaust gas is in an oxidizing atmosphere and so that the exhaust gas temperature is within a predetermined temperature range; and   contacting the exhaust gas, after its atmosphere and temperature are adjusted, with an NH 3  decomposing catalyst which, in an oxidizing atmosphere, resolves NH 3  when the temperature of the catalyst is within the predetermined temperature range, converts NH 3  in the exhaust gas in an oxidizing atmosphere to NO x  when the temperature of the catalyst is higher than the predetermined temperature range, and allows NH 3  to pass through the catalyst when the temperature of the catalyst is lower than the predetermined temperature range.   
     
     
       3. A process according to claim 2, wherein the internal combustion engine is operated at an air-fuel ratio close to the stoichiometric air-fuel ratio, and wherein a three-way reducing and oxidizing catalyst is used as the NH 3  synthesizing catalyst. 
     
     
       4. A process according to claim 2, wherein internal combustion engine is operated at an air-fuel ratio lower than the stoichiometric air-fuel ratio, and a three-way reducing and oxidizing catalyst is used as NH 3  synthesizing catalyst. 
     
     
       5. A process according to claim 2, wherein, after the exhaust gas has contacted the NH 3  synthesizing catalyst and before the exhaust gas has contacted the NH 3  absorbing catalyst, the exhaust gas is contacted with an NO x  absorbent which, in an oxidizing atmosphere, absorbs NO x  components. 
     
     
       6. A process according to claim 2, wherein, after the exhaust gas has contacted the NH 3  synthesizing catalyst and before the exhaust gas has contacted the NH 3  decomposing catalyst, the exhaust gas is contacted with an NO x  reducing catalyst which, in an oxidizing atmosphere, selectively reduces NO x . 
     
     
       7. A process for resolving NO x  from a combustion exhaust gas using an NH 3  decomposing catalyst which, in an oxidizing atmosphere with NH 3  present, resolves NO x  when the temperature of the catalyst is within a predetermined temperature range, converts NH 3  to NO x  when the temperature of the catalyst is higher than said predetermined temperature range, and allows NH 3  to pass through the catalyst when the temperature of the catalyst is lower than said predetermined temperature range comprising: supplying NH 3  to a combustion exhaust gas; and   contacting the exhaust gas with the NH 3  decomposing catalyst in an oxidizing atmosphere and at a temperature within said predetermined temperature range.   
     
     
       8. A process according to claim 1, wherein said NH 3  decomposing catalyst contains an NH 3  adsorbing component which adsorbs NH 3 . 
     
     
       9. A process according to claim 2, wherein said NH 3  decomposing catalyst contains an NH 3  adsorbing component which adsorbs NH 3 . 
     
     
       10. A process according to claim 7, wherein said NH 3  decomposing catalyst contains an NH 3  adsorbing component which adsorbs NH 3 . 
     
     
       11. A process according to claim 8, wherein said NH 3  adsorbing component adsorbs NH 3  when the temperature is lower than said predetermined temperature range. 
     
     
       12. A process according to claim 9, wherein said NH 3  adsorbing component adsorbs NH 3  when the temperature is lower than said predetermined temperature range. 
     
     
       13. A process according to claim 10, wherein said NH 3  adsorbing component adsorbs NH 3  when the temperature is lower than said predetermined temperature range.